1. Field of the Invention
The present invention relates to an ink for ink jet recording and an ink set comprising the same.
2. Background Art
An ink jet recording system is a method wherein an ink is ejected as droplets through fine nozzles and deposited on a recording medium to record thereon letters, figures, and images.
Examples of the ink jet recording system include methods, which have been put to practical use, such as a method wherein an ink is ejected utilizing vibration of a piezoelectric element and a method wherein an ink is ejected utilizing foaming created by rapidly applying heat to the ink. In recent years, the ink jet recording method has rapidly spread because use of a thermal transfer, sublimation, or laser method has offered advantages including easy formation of color images, low cost, and low running cost.
Inks used in such ink jet recording are required to have the following various properties according to the nature of the recording methods.
(1) High-quality images should be yielded on specialty paper (coated paper, gloss paper, films, and clothes treated for use in ink jet recording), as well as on plain paper (paper not subjected to any special surface treatment and including copying paper, reporting paper, bond paper, and letter paper, generally used in offices, schools, households and the like).
(2) The ink should be rapidly fixed, making it possible to realize high-speed printing.
(3) The ink should be less likely to clog nozzles and hence should be stably ejected without droplet trajectory directionality and dropout problems.
(4) The ink should realize images possessing excellent lightfastness and waterproofness.
(5) The ink should have good long-term storage stability.
In particular, for the requirement (1), an ink composition is demanded which can realize an image having no significant feather or bleeding on plain paper. There are various types of plain paper which are different from one another in surface state (density, length, penetration and the like of paper fibers). Therefore, feathering or bleeding is created in paper having good ink penetration. On the other hand, in paper having poor ink penetration, the drying of the printed image is so slow that, at the time of color printing, inks create color to color bleeding at boundaries therebetween, resulting in deteriorated print quality.
For the requirement (4), utilization of a pigment, such as carbon black, has been attempted from the viewpoint of waterproofness and lightfastness of printed images. For ink compositions for ink jet recording, water is used as a main solvent from the viewpoint of safety. Therefore, in order to provide a good ink composition for ink jet recording, a pigment, which is essentially insoluble in water, should be stably dispersed in water.
On the other hand, International Patent Publications WO 96/18688, WO 96/18689 and WO 96/18696 disclose a modified carbon black which has been rendered stable water-dispersion. In the present specification, the modified carbon black described in the above publications will be hereinafter referred to simply as "modified carbon black." These publications do not disclose any advantage associated with the use of the modified carbon black as a colorant for an ink composition for ink jet recording.